1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method for monitoring a control channel in a wireless communication system, and a wireless device using the method.
2. Related Art
Long term evolution (LTE) based on 3rd generation partnership project (3GPP) technical specification (TS) release 8 is a promising next-generation mobile communication standard. Recently, LTE-advanced (LTA-A) based on 3GPP TS release 10 supporting multiple carriers is under standardization.
A multiple input multiple output (MIMO) technique using a plurality of antennas is introduced also in a mobile communication system. 3GPP LTE supports up to 4 transmit antennas, and 3GPP LTE-A supports up to 8 transmit antennas.
The MIMO technique includes a spatial diversity scheme for increasing transmission reliability by transmitting a data symbol through various channel paths and a spatial multiplexing scheme for simultaneously transmitting a plurality of data symbols through a plurality of transmit antennas. In addition, the MIMO can be divided into single user-MIMO (SU-MIMO) and multi user-MIMO (MU-MIMO).
A capacity of a MIMO channel increases in proportion to the number of antennas. The MIMO channel can be decomposed into independent channels. If the number of transmit (Tx) antennas is Nt, and the number of receive (Rx) antennas is Nr, then the number of independent channels is Ni where Ni≦min{Nt, Mr}. Each independent channel may correspond to a layer. The layer may be defined as an information path which is input to a precoder. A rank is a non-zero eigenvalue of a MIMO channel matrix, and is equal to the number of layers or the number of spatial streams.
In 3GPP/LTE-A, a control channel does not support multiple antenna transmission. The control channel was designed under the premise that it is transmitted through a single antenna.
However, there is a need to consider transmission of a control channel through multiple antennas to cope with an increase in an amount of control information and to increase flexibility of scheduling.